The present invention relates to a vehicle steering apparatus.
In recent years, there has been proposed a steering control system provided with an active steering function determining a steering characteristic of a vehicle based on a detection of a state quantity such as a vehicle speed, a yaw rate or the like, and controlling the angle (tire angle) of wheels so as to control the yaw moment of a vehicle based on a result of determination.
For example, a vehicle steering apparatus described in Japanese Laid-Open Patent Publication No. 2005-262926 is provided with a variable transmission ratio device which changes a gear ratio corresponding to a transmission ratio between a steering wheel and vehicle wheels. The variable transmission ratio device changes the gear ratio mentioned above by superimposing an ACT angle, which is a second steering angle of the wheels based on the activation a motor, on a first steering angle of the wheels based on an operation of the steering wheel. In the case where a steering characteristic is oversteer, the apparatus controls the ACT angle so as to generate a steered angle in an opposite direction to the yaw moment. On the other hand, in the case where the steering characteristic is understeer, the apparatus changes the gear ratio mentioned above in such a manner as to reduce a changing amount of the steered angle with respect to a turning operation of the steering wheel. It is possible to stabilize the attitude of the vehicle by automatically controlling the operating angle of the wheels as mentioned above.
Conventionally, many of the steering apparatuses monitor the yaw rate of the vehicle and determine whether the active steering control needs to be started. However, under a condition where a friction coefficient μof a road surface is extremely low, such as a frozen road or the like, there is a tendency that the attitude of the vehicle becomes unstable in a moment of time due to even a small change of the steered angle or a change of a load. Accordingly, there may be a case in which the attitude of the vehicle has been already unstable at a time when the yaw rate corresponding to a threshold value for starting the active steering control is detected. In this case, there is a problem that a relatively long time is necessary until the active steering control is subsided, that is, until the attitude of the vehicle is stabilized.
Japanese Laid-Open Patent Publication No. 5-139327 discloses a method of determining that a turning operation of a vehicle reaches a limit and the attitude of the vehicle becomes unstable if a differential value of a slip angle of the vehicle, that is, the absolute value of an angular velocity of a side slip becomes equal to or greater than a threshold value. In accordance with this method, it is possible to instantaneously detect that the attitude of the vehicle is unstable, by monitoring the differential value of the slip angle.
However, there is a problem that a lot of erroneous determinations exist in the method of determining the attitude of the vehicle based on the differential value of the slip angle. In other words, in the case where the active steering control is started based on an erroneous determination, the attitude of the vehicle may be made more unstable. Accordingly, there is generally employed a method of determining when to start the active steering control based on the yaw rate.